This invention relates to drop-on-demand ink jet printing systems and more particularly, to a multi-color thermal ink jet printer adaptable to print in bidirectional swaths.
Color in documents is the result of combination of a limited set of colors over a small area, in densities selected to integrate to a desired color response. This is accomplished in many printing devices by reproducing separations of the image, where each separation provides varying density of a single primary color. When combined together with other separations, the result is a full color image.
Thermal ink jet printing is generally a drop-on-demand type of ink printing system which uses thermal energy to produce a vapor bubble in an ink filled channel that expels a droplet. A thermal energy generator or heating element, usually a resistor, is located in the channels near the nozzle a predetermined distance therefrom. The resistors are individually addressed with an electric pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink. As the bubble begins to collapse, the ink in the channel between the nozzle and the bubble starts to move toward the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum of the droplet which proceeds in a substantially straight line towards a recording medium, such as paper or transparencies, etc. (hereinafter, referred to as a xe2x80x9csheetxe2x80x9d). Sheets are moved past the printhead for creation of the image. Further details with respect to thermal ink jet printing devices are available at U.S. Pat. No. 4,638,337 to Torpey et al., and U.S. Pat. No. 5,422,664 entitled, xe2x80x9cMethod and Apparatus for Maintaining Constant Drop Size Mass in Thermal Ink Jet Printers,xe2x80x9d by J. Stephany which illustrate a multi-color ink jet printer.
In one type of ink jet device, a printhead including an array of several nozzles oriented parallel to the direction of sheet travel is moved across the recording medium in a direction perpendicular to the direction of sheet travel. Each transit prints a swath of spots or pixels across the page corresponding to an image. Most thermal ink jet printers print some number of scanlines in a swath, and the swath boundaries are visible in prints made thereby. The boundaries are visible because of ink flowing beyond the boundary of the array end (in the case of paper) or pulling away from the boundary of the array end due to surface tension (in the case of transparencies). In the former case a thin line along the boundary is more heavily inked; in the latter it is not inked at all.
It will be clear from the above description that printing can be accomplished in each transit of the head across the page, and for high speed black (or monocolor) only printing, bidirectional printing is common. Color ink jet printing is normally performed unidirectionally (left to right or right to left, not both). Multiple nozzle arrays arranged in parallel may be provided to print with multiple inks printing plural separations. One particular type of color ink jet printer has a single head (corresponding to an array of nozzles) per color, with four heads mounted one behind the other on the carriage, so that all four colors cyan, magenta, yellow and black of all scanlines in a swath are printed in a single pass, with a fixed order for the separations within a pixel. The reason bidirectional printing is normally avoided is that when the carriage is moving in reverse the colors are printed in reverse order. Printing a cyan pixel on a magenta pixel generally gives a different color from that when the printing order is reversed.
U.S. Pat. No. 4,999,646 to Trask discloses a unidirectional printing system, in which, to correct the interswath boundary artifact, a multiple pass complementary dot pattern ink jet printing process for enhancing the uniformity and consistency of dot (drop) formation during color ink jet printing is provided. Such enhancement in turn directly affects and improves the total print quality over a color printed area by minimizing the undesirable characteristics of coalescence, beading, hue shift, bending, cockling and color bleed when printing on both transparencies and plain or special papers. Using this process, successive printed swaths are made by depositing first and second partially overlapping complementary dot patterns on a print media. Simultaneously, the dot spacing in coincident dot rows within the overlapping portions of the dot patterns is alternated between dots in the first pattern and dots in the second pattern. Also note U.S. Pat. No. 4,748,453 to Lin et al. Various commercial products from Hewlett Packard Company, Palo Alto, Calif., including the HP500C, HP 550C and HPXL300 ink jet printers are believed to use a head signature mitigation pattern.
References disclosed herein are incorporated by reference for their teachings.
The present invention is directed to a method of using a color ink jet printer of the type which prints a swath of the image at a time with each transit across a sheet to print color images with a substantially reduced bidirectional printing artifact.
In accordance with one aspect of the invention there are provided method and apparatus for operating a color ink jet printer of the type which prints a swath of the image at a time with each transit across a sheet. An image may be optionally preprocessed with full undercolor removal, so that each color pixel in the image is defined by black and only two colored inks. Color will be deposited to print the image on forward and return transits of a four color printhead across a sheet. The colors and location printed in each transit are determined prior to the first transit, by determining first, which colors may be printed in the second transit irrespective of their order of deposit. The result of this determination is logically ANDed with a checkerboard pattern to determine printing locations. Then, the other required colors and locations for the image which will not be printed with the second transit are printed on a first transit. Each swath is printed with forward and reverse transits, with checkerboarding of the reverse transit to prevent head signature artifacts.
If only one color is printed, it can be done in a checkerboard fashion, with one parity of the checkerboard on the first pass, and the other on the second pass. If two colors are printed in one pixel, one of them is normally printed first. In the unidirectional mode this is totally specified; whichever color is printed first in unidirectional printing should be printed in the first pass for bidirectional printing. The second color can be checkerboarded, printing half in each pass.
If printing order KCMY is used for right-to-left printing on the printer, let X be a bitmap representing the checkerboard, and let C, M, Y and K be bitmaps representing the input. The first pass is derived from the second pass. For the second pass:
K2LR=KX
C2LR=C(MY)X
M2LR=MYX
Y2LR=YX
The subscript LR emphasizes that the direction of travel is left to right. The symbols ,  and  indicate the logical operations of AND, OR and NOT respectively. Then for the first pass,
K1RL=KK2LR
C1RL=CC2LR
M1RL=MM2LR
Y1RL=YY2LR
In the described manner, correct color printing can be accomplished with bidirectional printing, and with checkerboard prevention of head signature artifacts.